1. Field of the Invention
The present invention relates to a method of shaping hollow parts from synthetic resins, as well as a mold assembly for use in that method. In particular, the invention relates to a method and apparatus for shaping hollow parts with an injection molding machine.
2. Related Art
A conventional method of shaping hollow parts and a mold assembly for use in that method as shown in FIG. 14 is proposed (see Examined Published Japanese Patent Publication Hei 2-38377). This method and mold assembly are described below with reference to those figures.
FIG. 1 is a longitudinal sectional view showing an example of the mold assembly of the invention for shaping hollow parts. FIGS. 2 to 8 are longitudinal sectional views illustrating the method of shaping a hollow part with the mold assembly by showing its state in each step of the method.
As is clear from FIG. 1, the mold assembly generally indicated by numeral 1 is composed of a fixed mold 2, a sliding mold 3 and a movable mold 4. The fixed mold 2 is secured to a fixed platen 7 which is integral with the bed 6 of an injection molding machine 5. A support 8 having a horizontal arm 8a is erected on top of the fixed mold 2. The underside of the horizontal arm 8a is furnished with a hydraulically, pneumatically or otherwise operated sliding cylinder 9. The cylinder 9 has a piston rod 9a connected to the top of the sliding mold 3. Thus, the sliding mold 3, as it keeps adhering to the principal face of the fixed mold 2, is capable of sliding vertically between the lower position where the cylinder 9 is extended fully and the higher position where it is contracted fully.
The movable mold 4 is mounted on a movable platen 10 which is supported on the bed 6 of the injection molding machine 5 in a horizontally movable manner. The movable platen 10 is adapted to be moved back and forth with respect to the fixed platen 7 by means of a mold clamping mechanism not shown. Thus, the movable mold 4 is adapted to be moved back and forth between the mold registering position where it adheres to the sliding mold 3 and the mold opening position where it is remote from the sliding mold 3.
The fixed mold 2 has in its center a sprue 12 for guiding a molten resin injected from an injection unit 11 mounted on the fixed platen 7. The sliding mold 3 has a center sub-sprue 13 that is continuous to the sprue 12 when the sliding mold 3 is located in the lower position, as well as a lower sub-sprue 14 that is continuous to the sprue 12 when the sliding mold 3 shifts to the upper position.
The mold registering face of the sliding mold 3 has a male die 15 and a female die 16 provided in an upper and a lower position that are symmetric with respect to the center sub-sprue 13. The male die 15 will shape the inner surface of one of the split halves of the hollow part to be made, and the female die 16 will shape the outer surface of the other split half. The mold registering face of the movable mold 4 is provided with a female die 17 and a male die 18 that will face the male die 15 and the female die 16, respectively, when the sliding mold 3 is located in the lower position. The female die 17 will shape the outer surface of one split half whereas the male die 18 will shape the inner surface of the other split half. The female die 17 in the movable mold 4 is adapted to face the female die 16 in the sliding mold 3 when it is located in the upper position.
Thus, when the sliding mold 3 is located in the lower position and the movable mold 4 brought into registry with it, a pair of cavities 19 and 20 are formed between the sliding mold 3 and the movable mold 4, the cavity 19 being defined by the male die 5 and the female die 17 and the cavity 20 by the male die 18 and the female die 16. In this case, the center sub-sprue 13 in the sliding mold 3 communicates with the end edge portion of the female die 17 (or 16), namely, the cavity 19 (or 20) via a runner 21 and a gate 22 (or 23) that are formed in the movable mold 4. On the other hand, when the sliding mold 3 is located in the upper position and the movable mold 4 brought into registry with it, the female dies 16 and 17 in the sliding mold 3 and the movable mold 4, respectively, are brought into abutment against each other so that the lower sub-sprue 14 will communicate with the end edge portions of these female die 16 and 17 via the runner 21 and the gate 22 in the movable mold 4.
The peripheral edge portions of the male dies 15 and 18 are furnished with small projections 24 and 25 that are to be fitted in the peripheral edge portions of the female dies 17 and 16, respectively.
The process of shaping a hollow part using the mold assembly 1 proceeds as follows. First, the cylinder 9 is extended to locate the sliding mold 3 in the lower position. Then, the movable platen 10 of the injection molding machine 5 is moved toward the fixed platen 7 so that the movable mold 4 is in registry with the sliding mold 3. Then, as shown in FIG. 1, the center sub-sprue 13 in the siding mold 3 is continuous to the sprue 12 in the fixed mold 2, forming a pair of cavities 19 and 20 between the sliding mold 3 and the movable mold 4.
In the next step, a molten resin is injected from the injection unit 11 mounted on the fixed platen 7. The injected resin passes through the sprue 12 in the fixed mold 2 and the center sub-sprue 13 in the sliding mold 3 to be guided to both cavities 19 and 20 through the runner 21 and the gates 22 and 23, filling those cavities 19 and 20 as shown in FIG. 2. Thus, two split halves 31 and 32 of the desired hollow part are shaped in the cavities 19 and 20, respectively.
After the split halves 31 and 32 are cooled to solidify, the mold clamp unit is operated to detach the movable mold 4 from the sliding mold 3 as shown in FIG. 3. Then, the male dies 15 and 18 are disengaged from the split halves 31 and 32, which will remain in the female dies 17 and 16, respectively. In this mold opening mode, a resin sprue runner portion 33 that has solidified within the sprues 12, sub-sprue 13, runner 21, etc. in the mold assembly 1 is ejected therefrom and falls free as a result of separation in those areas of the portion 33 which correspond to the gates 22 and 23. The thus shaped split halves 31 and 32 have their end faces serve as abutting faces 31a and 32a which are to be brought into abutment against each other. Grooves 31b and 32b have been shaped in the peripheral edges of the respective abutting faces 31a and 32a by means of the projections 24 and 25 around the male dies 15 and 18.
Subsequently, as shown in FIG. 4, the cylinder 9 is contracted to shift the sliding mold 3 to the upper position. Then, the female die 16 in the sliding mold 3 faces the female die 17 in the movable mold 4 so that the split half 32 left in the female die 16 will face the split half 31 in the female die 17. In this case, the lower sub-sprue 14 in the sliding mold 3 is continuous to the sprue 12 in the fixed mold 2.
The movable mold 4 is then moved toward the sliding mold 3 so that the two members are in registry with each other as shown in FIG. 5. As a result, the abutting faces 31a and 32a of the respective split halves 31 and 32 are brought into abutment against each other, with the grooves 31b and 32b forming a space around the abutment. This space communicates with the sub-sprue 14 via the gate 22 and the runner 21.
If another shot of molten resin is injected from the injection unit 11, the injected resin passes through the sprue 12 in the fixed mold 2, the lower sub-sprue 14 in the sliding mold 3 and through the runner 21 and the gate 22, filing the peripheral edge portion of the abutment of the split halves 31 and 32 as shown in FIG. 6. The resulting peripheral resin 34 allows the two split halves 31 and 32 to fuse together.
After the peripheral resin 34 is cooled to solidity, the mold clamp unit is operated again to detach the movable mold 4 from the sliding mold 3. A hollow part 30 in which the two split halves 31 and 32 have been fused together in an abutment relationship to be completed as a totally sealed part is recovered from the mold assembly 1. In this case, a resin sprue runner portion 35 that has solidified in the sprue 12, sub-sprue 14, runner 21, etc. in the mold assembly 1 is separated from those areas of the portion 35 which correspond to the gates 22 and 23.
After recovering the hollow part 30 thusly, the cylinder 9 is extended again as shown in FIG. 8 so that the sliding mold 3 is located in the lower position. If the movable mold 4 is brought into registry with the sliding mold 3, the process returns to the first mode shown in FIG. 1 for starting the shaping of another part.
By repeating this sequence of steps, a plurality of hollow parts 30 can be shaped continuously. Since the overall shaping process is composed of simple steps (i.e., sliding the mold 3 vertically, mold registering and opening by moving the mold 4 back and forth, and injecting a molten resin), fully automation of the process can be easily accomplished. This enables the mass-production of hollow parts 30.
In the process, the two split halves 31 and 32 are first injection molded separately and then fused together in an abutment relationship; hence, the process allows for such a great degree of freedom in the wall thickness and shape of the final part 30 that even a completely sealed hollow part can be shaped. In addition, the need for deburring is substantially eliminated from the process. Further, the split halves 31 and 32 are fused by means of the mold assembly 1 and the injection unit 11 that are used to shape them and, hence, not only is the fusing step simplified but also satisfactory fusion strength can be attained.
In the example described above, it is assumed that the sliding mold 3 and the movable mold 4 form a pair of cavities 19 and 20, from which one hollow part 30 is shaped. If desired, more than one pair of cavities 19 and 20 may be formed so that a plurality of hollow parts 30 can be shaped simultaneously.
The example also assumes that the sliding mold 3 is slid vertically along the fixed mold 2. If the male dies 15 and 18 are adapted to be capable of separation or retraction, the sliding mold 3 can be designed as one that rotates around the center sub-sprue 13 to slide along the fixed mold 2. If this arrangement is adopted, only the center sub-sprue need be provided in the sliding mold 3.
Being composed in the manner described above, the prior art injection molding has had the following problems.
The shaping condition control and setting unit stores a single set of shaping conditions and, if the molds to be used alternately for shaping have the same cavity geometry, the single set of shaping conditions will suffice; however, if the molds have different cavity geometries (as in the case of molding those parts of an automotive headlamp on the right side which are symmetrical with those of an automotive headlamp on the left side), the single set of shaping conditions which may be satisfactory for one mold is certainly inappropriate for the other mold and it has been impossible to yield a shaped part that satisfies fine and subtle conditions.
Further, the conventional art method of shaping hollow parts is capable of economical mass production but, on the other hand, it employs only one injection cylinder and, hence, it has been unable to meet the requirement for changing the colors or materials of a pair of shaped halves so that one can identify the contents of the final shaped part from outside.
Further, it was found that when injecting a molten resin during secondary shaping for joining the pair of split halves, holes would potentially be made in the wall surface of the hollow part on account of the pressure or heat of the molten resin in the neighborhood of the gate, thereby increasing the chance of the molten resin of leaking through the holes to get into the hollow part. Another potential problem is that the split halves may deform under an increased injection pressure. If the injection pressure is reduced in an attempt to prevent this phenomenon, the resin will not be supplied adequately to the abutting faces of the two split halves, thereby increasing the likelihood of "short molding". A solution to the problem of "gate cutting" will facilitate the automation of the overall shaping process but, in fact, the problem of "gate cutting" persists.
The resin sprue runner portions and the shaped hollow part are recovered by pushing into the mold assembly an ejector that is provided on the side of the movable platen on which the movable mold is mounted. In general-purpose injection molding machines, the ejector is provided in one location on the side of the movable platen which faces the sprue in the mold assembly. In the production of a hollow part by the prior art technique described above, the ejector is actuated twice, the first time for recovering one resin sprue runner portion and the second time for recovering the shaped hollow and the other resin sprue runner portion. Since the conditions for the first recovery differ from those of the second recovery, problems have been encountered with the method and apparatus for these recoveries.
In certain cases, one may want to equip a single unit of injection molding machine with various kinds of molds to shape different kinds of hollow parts but the ejector problem makes it impossible to meet this need.
Further, with the recent trend for the sophistication of automobiles and other engineering products, the conditions for shaping fine details which have heretofore been entirely left out of consideration have become influential on the value of engineering products that use the parts shaped under those conditions. This situation cannot effectively be dealt with by the conventional shaping condition setting unit which is only capable of providing for a single set of conditions.
On the other hand, in the case, when an insert is provided within a hollow part during one step of the shaping process and, at the same time, adapting the pair of shaped halves to have different colors or to be made from different materials, thereby enabling one to identify or otherwise recognize the contents of the final shaped part from outside.
With hollow parts of the type contemplated by the present invention, it has been a common practice to have a shaped insert fitted in manually. To shape a single hollow part using a pair of shaped halves made either from dissimilar materials or in different colors, the method shown in FIG. 14 is typically employed. Shown by numeral 301 in FIG. 14 is the process of making a first shaped half 301a. The process 301 starts with step 301A of first mold closing and goes through step 301B of first injection and dwelling, step 301C of first cooling and plasticizing and step 301D of first mold opening and ends with step 301E of first recovery, thereby yielding the first shaped half 301a.
Sown by 302 in FIG. 14 is the process of making a second shaped half 302a. The process 302 starts with step 302A of second molding closing and goes through the 302B of second injection and dwelling, step 302C of second cooling and plasticizing and step 302D of second mold opening and ends with step 302E of second recovery, thereby yielding the second shaped half 302a.
Shown by 303 in FIG. 14 is the process in which the two shaped halves 301a and 302a are combined and a binder resin 303b is injected around the mating faces to make a single hollow part 303a. The process 303 starts with step 303A of third mold closing and goes through step 303B of third injection and dwelling, step 303C of third cooling and plasticizing, step 303D of third mold opening and step 303E of third recovery and ends with step 304 of inserting. Thus, the two shaped halves 301a and 302a are made integral to yield the hollow part 303a.
The conventional art method which is shown in FIG. 14 must use three injection cylinders and, hence, the production efficiency is too low to be suitable for achieving economical mass production.